1. Field of the Invention
This invention relates to a body frame for a motorcycle that is provided as a cast article made of a light alloy, and more particularly to a body frame of the type wherein an engine is used as a reinforcing member for the body frame. A hollow body frame is designed to reduce the production of noise. Further, a seat frame is provided that is formed as a unitary member.
2. Description of Background Art
Body frames for a motorcycle normally include a pipe frame wherein a plurality of pipe members are assembled as a unitary member by welding. A steel plate frame is also available wherein a pair of left and right steel plate half bodies, formed by press work, are joined to each other by welding such that a principal section of the frame may be a hollow section. Further, a composite frame of a steel pipe and a steel plate is also available.
A hollow body frame for a vehicle having a small size is formed from a steel pipe, a protruded light alloy pipe, a cast light alloy pipe or the like. Hollow body frames of this particular type are apt to generate resonance sounds at a location at which a noise producing body such as an engine or a motor is supported due to vibrations transmitted at these locations.
In the case of a pipe frame which is used widely, it is necessary to attach various projecting pieces for supporting members thereon to the pipe frame by welding. Further, reinforcing lugs are used for mutually coupling portions of pipe members. Prior to assembly, each pipe member must be drawn to provide a desired sectional shape to a location thereon and must be suitably bent to provide the same with a curved configuration. In addition, since each of the pipe members has a fixed sectional area, a reinforcing member must be applied to a location of the pipe member upon which a heavy load acts. Accordingly, in order to obtain a conventional welded pipe frame, a large number of parts and a large number of working man-hours are required.
To the contrary, where a body frame is formed by casting as disclosed in Japanese Patent Laid-Open No. 61-6084 and Japanese Patent Laid-Open No. 62-31576, reinforcing ribs and supporting projecting pieces for various parts can be provided at the time of casting, and the number of working man-hours can be reduced significantly.
The inventors have made sufficient investigations over advantages in the production of a body frame by casting. It has been determined that not only a reduction in the number of working man-hours can be anticipated but also an optimum configuration which takes a load application into consideration can be adopted for a body frame of a particular type.
If a body frame made of a light alloy is to be constructed as a pair of left and right divided half bodies or a unitary block by casting, it is very difficult to form a long size hollow portion having a thickness of material less than 4 mm to the same degree as a pipe material which is used for assembly of a known body frame made of an aluminum alloy.
Normally, a body frame constructed of an aluminum alloy is made as a large size cast article and is produced by gravity die casting. If an attempt is made to restrict the thickness of the material of a long size hollow portion to less than 4 mm for reduction in weight, the speed of molten bath in a dam that communicates and connects a runner and a cavity with and to each other is about 20 to 30 cm/sec by normal gravity die casting. Thus, the speed of the molten bath flowing into the cavity is insufficient so that the molten bath cannot fill the cavity. Thus, while it may seem recommendable to arrange the cavity in an inclined posture so that molten bath may be poured at a high molten bath speed through a dam communicating with an upper end portion of the cavity, the molten bath having passed the dam preferentially flows to corner portions of the cavity at a high speed due to the action of surface tension and thus closes an opening of the air exhaust path to prevent air within the cavity from escaping through the air exhaust path. Consequently, an article of a desired configuration cannot be obtained.
According to low pressure die casting, the speed at which molten bath passes a dam can be made higher than 50 cm/sec. However, the molten bath having passed the dam is advanced into a cavity like a jet so that such a phenomenon appears that a large amount of air, air bubbles, remain in a cast article. Also, solidification proceeds rapidly due to the fact that the cast article has a thin wall. Consequently, a good product having a desired strength and a desired extension characteristic cannot be obtained.
Further, while a technique is also known wherein a metal mold is preheated to remove remaining air bubbles to delay the solidifying speed of the molten speed, such a technique is cumbersome in that the temperature management of the metal mold must be strictly controlled.
A cast frame having two divided blocks is conventionally known and disclosed for example, in Japanese Patent Laid-Open No. 61-129383. According to the cast frame, the motorcycle body is composed of a head pipe portion and a body frame portion extending downwardly and rearwardly in an inverted U-shaped shape. The head pipe portion is formed by casting into two symmetrical left and right blocks including a head pipe along a dividing plane on a center line of a car body. The two blocks are coupled into a unitary member by a bolt connection and various stays are mounted thereon.
In a motorcycle frame formed by casting as two divided blocks, a front portion thereof is secured by means of a bolt by way of a flange formed on a head pipe portion. A rear portion thereof is secured by means of a bolt and a cross member. Although the car body frame is formed by casting, the number of man-hours for machining steps with regard to bolt holes, and assembling steps, such as assembly of the cross member and several stays, is increased. This leads to deterioration in productivity. In addition, the head pipe portion is formed into two divided portions, the accuracy at the head pipe portion cannot be attained and the strength of the head pipe portion is low.
Further, a seat frame extends rearwardly from an upper intermediate portion of the frame body, the number of man-hours for assembly is increased. Thus, productivity deteriorates because several stays and a mud guard are mounted on the seat frame.
The present invention utilizes products of press work for the seat frame, cross member, mud guard, several stays, etc. the strength of these members takes precedence. Some waste may appears in the shape of the members so that the entire motorcycle frame is increased in weight.
A seat frame, for example, as disclosed, in Japanese Patent Laid-Open No. 62-96185 is known for replacing a conventional seat frame that is constructed of welded pipes. The seat frame is constructed such that a pair of left and right frames of an inverted triangular truss construction serve as a pair of left and right seat rails and back stays which are mounted on and extend rearwardly form an intermediately upper portion of a main frame. The frames are formed by press work and are secured to each other by means of bolts using three cross members extending across upper faces of the two frames. Mounting bosses for various parts are welded to the two left and right frames and various brackets, stays, etc. are mounted on the mounting bosses. A spacing below the cross members is surrounded by the two left and right frames. The cross members are made as a spacing for rocking motion of a rear wheel.